That's easy, because most people do not care to do that. Having owned a collection of DVDs it was "fun" to buy them when it was the thing to do but totally useless now. I care not to take the baggage of managing hundreds of titles with me just to have the knowledge that I own it. I do not wish to own such things unless I want to - which will be rare and should be on me to purchase that item outright from the provider.
This is about simplicity and ease-of-use only, not about ownership. If you insist on ownership there is nothing preventing you from buying the work directly.
I'm fine with buying the work directly, but can you point to a good store which sells it simply as files with no DRM attached which I can download after buying? I don't need physical DVDs clutter, it's so much last century.
Sure, streaming as convenience is good to have. But it doesn't remove the need for personal backup. Network connectivity is not available always and everywhere, as well as you can't rely on the service to be always around. If you don't care to lose your extensive digital library when that service goes bust it's up to you, but it's a normal thing to care about it.
This is about simplicity and ease-of-use only, not about ownership. If you insist on ownership there is nothing preventing you from buying the work directly.